1. Field of the Invention
The present invention relates to a method of manufacturing an alkali metal salt of 2,3,6,7-naphthalenetetracarboxylic acid.
2. Description of the Related Art
2,3,6,7-naphthalenetetracarboxylic acid, having a symmetrical structure, like pyromellitic acid, may be useful as raw materials for various polymers and synthetic materials of plasticizing agents, dyes, or pigments.
This carboxylic acid is described in Y. Dozen, Thermochim. Acta. 25, 209-216, 1978. However, the method of preparing the acid, which is disclosed in this thesis, includes many synthesizing steps, and no methods which can be industrially satisfactory have been completed.
On the other hand, methods of manufacturing 1,3,5,7-naphthalenetetracarboxylic acid, which is one isomer of the naphthalenetetracarboxylic acids, are disclosed in several publications such as Published Unexamined Japanese Patent Application Nos. 49-102656, 49-102657, and 51-26857. Any of these methods utilizes rearrangement or disproportionation reaction of aromatic carboxyl groups, which is generally known in the art as the Henkel reaction.
The Henkel reaction is to form an aromatic dicarboxylic acid by heating an aromatic monocarboxylic acid alkali metal salt and/or a polycarboxylic acid alkali metal salt to a high temperature in an atmosphere of carbon dioxide in the presence of a catalyst such as an oxide of cadmium or zinc. This reaction has long been utilized to manufacture dipotassium terephthalate from potassium benzoate or dipotassium phthalate, and to manufacture dipotassium 2,6-naththalenedicarboxylate from potassium naphthoate or potassium naphthalate, for example. In the prior art Henkel reaction, the alkali of the alkali metal carboxylate or the alkali halide used as co-catalyst is potassium in most cases. In fact, potassium better serves to manufacture the target product at a high yield, than any other alkali. For example, when sodium salt is used in place of potassium salt, more by-product is formed, inevitably reducing the yield of the target product. In the prior patent applications relating to the Henkel reaction, though the alkali is not limited to a potassium salt in the claims of these patent applications, only a potassium salt is cased in the working examples thereof in these examples.
Published Examined Japanese Patent Application No. 36-13629 discloses a method of manufacturing terephthalate from mixed sodium and potassium phthalates, which mixed phthalates have a K/Na atomic ratio of 95/5 to 70/30. In this method, by using sodium salt in a small amount, the melting point of the mixed phthalates is lowered below that of the dipotassium phthalate, so that a lower temperature is sufficient to melt the phthalate, making it possible to produce the terephthalate at a high yield. However, the main component of the alkali is potassium in this method. The sodium salt is used to lower the melting point of the phthalate raw material and in an amount such that the yield of the terephthalate is not lowered.
A method of manufacturing pyromellitate by means of the Henkel reaction is disclosed in Published Examined Japanese Patent Application No. 45-656 in which one of the present inventors is also named as an inventor. In this method, a mixture of sodium phthalate and potassium phthalate, which has a Na/K atomic ratio of 95/5 to 70/30, is heated to a high temperature in an atmosphere of an inert gas such as carbon dioxide gas, in the presence of a Henkel reaction catalyst. This publication teaches nothing about using of a co-catalyst in the Henkel reaction.
The carboxylic acid used as the material in this method is a benzene series, whose melting point is relatively low. Therefore, the temperature to which the mixture of said salts must be heated to accomplish the Henkel reaction is comparatively low. Because of the low temperature, the carboxyl group is scarcely decomposed by heat before the material (i.e., carboxylic acid) undergoes the Henkel reaction. As a result, pyromellitic acid is produced at a high yield. Further, since the carboxylic acid used as the material, which is a benzene series, has a few positions where the carboxyl group can be substituted, the produced positional isomers are small in numbers. It is therefore relatively easy to isolate pyromellitic acid from the other reaction products, whereby pyromellitic acid can be obtained which has a high purity.
When this method is employed to process naphthalene-series carboxylate, however, it is difficult to manufacture 2,3,6,7-naphthalenetetra-carboxylic acid, for the following reason.
Most naphthalene-series compounds have a melting point higher than that of benzene-series compounds. Hence, to subject naphthalene-series carboxylate to the Henkel reaction successfully, it is necessary to heat this material to a temperature higher than a benzene-series carboxylate. Therefore, the carboxyl group is likely to be thermally decomposed before the naphthalene-series carboxylate is heated to the reaction temperature. Consequently, 2,3,6,7-naphthalenetetracarboxylic acid is obtained at a low yield. Since the carboxylic acid used as the raw material is a naphthalene series and has many positions where the carboxyl group can be substituted, the produced positional isomers are proportionally great in numbers. It is inevitably difficult to isolate 2,3,6,7-naphthalenetetracarboxylic acid from the other reaction products. Hence, the 2,3,6,7naphthalenetetracarboxylic acid, thus obtained, has but low purity.
A method other than the Henkel reaction can be used to manufacture 2,3,6,7-naphthalenetetracarboxylic acid. This method, however, includes many synthesizing steps, and can not achieve a high yield of 2,3,6,7naphthalenetetracarboxylic acid. Therefore, the method can hardly be employed on industrial scale.